Vertical Takeoff and Landing (vertical lift) aircraft have long been considered desirable because of their ability to hover in flight and transition in and out of flight without a runway, in addition to flying in a horizontal direction. Although rotating-wing vertical lift aircraft (helicopters) have long been available, a rotating wing requires substantial clearance and can present safety hazards. In addition, rotating-wing aircraft generally have poor cruise performance compared to fixed-wing aircraft. Consequently, other types of aircraft, for example lifting off in a “tail sitting” configuration or employing “fan in wing” structure, are considered preferable in many situations.
During stationary flight, a rotating-wing vertical lift aircraft is supported by lift from air flow across its wing. Because the lift is developed across a relatively wide area, rotating-wing vertical lift aircraft possess some inherent stability against roll. Vertical lift aircraft other than rotating-wing aircraft do not enjoy such lateral stability because they are supported by a relatively compact source of thrust. For example, a ducted-fan type of vertical lift aircraft may be viewed, in operation, as sitting on a column of air. Although multiple thrusters can be employed for additional lateral stability, such an arrangement adds complexity and presents similar size disadvantages to those of a rotating-wing vertical lift aircraft.
Also, certain conventional types of fixed-wing vertical lift aircraft are capable of transitioning between vertical flight and horizontal flight while part or all of the vehicle transitions between a vertical and horizontal orientation with respect to the ground. Conventional approaches are problematic, however, when it comes to accommodating a payload while the vehicle makes the transition. Conventional methods for orienting the payload with respect to the rotating vehicle generally can be grouped into two broad categories: fixed payload and mechanically rotated payload.
Permitting the payload to rotate with a vertical lift aircraft from a horizontal to vertical orientation is generally undesirable. If the payload performs ground observation such as monitoring ground-based targets or tracking a vertical landing site, for example, compensations must be made while transitioning between horizontal and vertical orientations. If the payload includes humans, they must deal with the discomfort of moving between sitting and lying positions. These deficiencies have a compound effect when a pilot attempts to land vertically or visually track ground-based targets because of the combined disorientation and discomfort they cause.
Some conventional vertical lift aircraft have been developed in which all or part of the vehicle mechanically rotates with respect to the payload, thus permitting the payload to remain in a substantially fixed orientation while the rest of the aircraft rotates. When viewed from the perspective of the rotating portion of the aircraft, it is the payload that is mechanically rotated. A broad class of such vertical lift aircraft configurations can be categorized as having a mechanically rotated payload, including tilt rotor, tilt duct, and tilt wing. This class of aircraft presents a serious control problem, in that the mechanical rotations tend to be destabilizing and must be carefully coordinated with aerodynamic controls to keep the vehicle airborne. Furthermore, the mechanism needed to affect the rotation tends to be heavy, which adds weight and reduces aircraft performance. In addition, the mechanism represents a single failure point with potentially destabilizing failure modes. When considered together with the above problems, the typical complexity of the mechanism can present a serious safety hazard.
In view of the many problems associated with conventional rotating-wing and fixed-wing vertical lift aircraft, it would be desirable to have a new type of vertical lift aircraft.